When a test fails the first thing I do is look in tests.log, that will
still work just as well with the combined test. I suppose we do lose
out if there are multiple independent bugs, as the test will stop at the
first one and not report the others.

I feel we should be optimising for the common case where the tests PASS,
and that optimising for FAILs is the wrong approach.

I agree that combining big, complex tests, like merge, is a bad idea.
But for relatively trivial tests I think reduced runtime is more
important.